Does the National Electrical Code (NEC) require all lighting installations operating at 30 volts or less to be Listed systems? Can individual low-voltage lighting components, such as a power supply, fixtures, and conductors be assembled in the field without being part of a Listed system?

These questions need to be answered before designing or installing low-voltage lighting for residential or commercial outdoor landscapes or interior ambient lighting systems.

Low-voltage lighting has been used for many years in gardens, around walkways, decks, or patios, and as accent lighting for trees and other shrubbery. The relatively low cost and the ease of installation have increased use of systems in recent years for both commercial and residential occupancies.

Despite literally millions of low-voltage lighting systems installed in the United States, there wasn’t an article in the NEC where an installer could look for specific criteria covering low-voltage lighting systems until 1996.

Article 720 was inserted into the NEC in the early days of electrical power to cover low-voltage systems installed for farms and rural buildings where normal electrical power was not available. With the advent of the Federal government’s Rural Electrification Administration (REA) program, power lines connected the power company grid to rural America. Even though Article 720 is still in the NEC, it would have been difficult to fit low-voltage landscape and interior low-voltage lighting into this existing article.

It would have been equally difficult to incorporate coverage of low-voltage lighting into Articles 725 or 410. For this reason, a new Article 411 was developed for the 1996 NEC. This new article provided a single location to access the installation requirements for low-voltage lighting Listed as a complete system.

The criteria necessary for a lighting system to be covered by Article 411 was a power supply that operated at 30 volts or less with a maximum peak available voltage under any load condition of 42.4 volts. The power source could consist of one or more secondary circuits, but each could not exceed 25 amperes.

These secondary circuits must be insulated from the primary of the power supply (branch circuit) by an isolating transformer. A branch circuit larger than a maximum 20-amperes cannot supply the primary of the isolation transformer. Assuming a 120-volt primary with a 30-volt secondary, the ratio of the transformer would be 4-to-1.

Using the maximum of the 20-ampere primary, the secondary current could be as much as four times or a total of 64 amperes (4 x 20 amps x 80 percent for continuous use) at 30 volts. Remember that each secondary circuit cannot exceed 25 amperes, so additional secondary protection must be installed. Three 20-ampere circuits, four 15-ampere circuits, or any other combination could be used as secondary circuits to supply low-voltage lighting fixtures (called luminaires in the 2002 NEC).

If the low-voltage lighting system is a landscape lighting installation, the installation must comply with the requirements in Section 300-5 and its accompanying table. SPT-3, UF, or other identified cable or raceway must be used for the secondary circuits. The burial depths shown in Column 5 of Table 300-5 are required for the various wiring locations for the wiring method supplying the low-voltage fixtures.

The circuit feeding the primary side of the low-voltage transformer is a branch circuit. Sections 210-19(a) and 210-20(a) require that branch circuit conductors and overcurrent protection devices, sized and rated as a continuous load, be calculated at 125 percent. The conductors on the secondary side must be sized based upon the manufacturer’s instructions and the load on each set of conductors, but, since these are no branch circuit conductors, sizing the conductors at 125 percent is unnecessary.

Voltage drop on a low-voltage system is critical in permitting the lighting system to operate properly, therefore conductor size plays a very important role and should be taken into consideration, driving the design of any lighting system.

The total area to be covered and the size and number of lights on the low-voltage system may require a larger system than permitted for low-voltage lighting systems covered by Article 411. Individually Listed low-voltage components could still be assembled to cover a large low-voltage lighting system, but would not be covered by Article 411.

The transformer must be protected in accordance with Section 450-3(b), but would not be limited to a maximum of a 20-ampere branch circuit. The secondary circuits would not be limited to 25 amperes. The number of low-voltage lighting fixtures and the size of the conductors would be limited, based upon the size of the overcurrent protection provided on the secondary side of the transformer.

This installation would be considered a Class 1 power-limited circuit and must comply with the requirements in Parts A and B of Article 725. The fixtures must comply with Article 410 and the wiring system must comply with Chapter 3.

The important point to remember about low-voltage lighting systems covered by Article 411 is that the lighting is Listed as a system, as opposed to low-voltage lighting that is assembled as component parts.

ODE is staff engineering associate at Underwriters Laboratories, Inc., in Research Triangle Park, N.C. He can be reached at (919) 549-1726 or via e-mail at mark.c.ode@us.ul.com.